The present invention relates to a needle selection device for circular knitting machines.
As known, circular knitting machines comprise a needle cylinder with a skirt in which a plurality of grooves is defined; said grooves extend parallel to the axis of said skirt, and each accommodates a needle and a selector or subneedle. When forming stitches of knitting, the needles are actuated with an alternating motion along their related grooves by means of cams, arranged around the needle cylinder which engage with a needle heel protruding radially from the grooves, and which, upon rotation of the needle cylinder about its own axis relatively to said cams, cause this alternating motion. In order to determine which needles must knit, according to the various knitting requirements, selection devices are used which act on the selectors so as to engage or prevent the engagement of a main heel thereof with a lifting cam which, if it engages said main heel, moves the selector to act on the overlying needle to engage its heel in a preset path defined by the related cams for causing it to knit.
Selectors generally have, below their main heel, one or more selection heels located at different heights. The selectors are accommodated in the related grooves of the needle cylinder and can oscillate in a radial plane towards or away from the axis of the needle cylinder so as to pass from an inoperative position, in which their main heel is completelY inserted into the related groove of the needle cylinder and therefore does not engage with the lifting cam when the overlying needle is to be excluded from knitting, to an operative position, in which their main heel protrudes radially from the related groove to engage with the lifting cam when the overlying needle must knit. In order to oscillate the selectors and therefore move them from their operative position to their inoperative position or vice versa, selection devices are generally used which comprise a plurality of selection levers arranged upstream cf the lifting cam, according to the direction of rotation of the needle cylinder Each selection lever has one of its ends directed towards the needle cylinder and arranged substantially at the level of one of the selection heels of the selectors, and can be controllably moved from a first position, in which it interferes with one or more selection heels to oscillate the related selectors, to a second position in which it does not interfere with the selection heels of the selectors.
The selection levers are moved from the first position to the second and vice versa by oscillating the selection levers or by moving the selection levers towards or away from the axis of the needle cylinder. In some selection devices this movement is achieved by mechanical actuation means, while in others it is achieved by electromagnets which are energized or de-energized according to the knitting program of the machine.
In any case, in order to obtain a great possibility of selection it is necessary to use selectors with a large number of selection heels, which are therefore relatively long. In the case of machines having a large number of needles per unit of length, i.e. with low-thickness selectors, the disadvantage may occur that the selectors flex longitudinally, reducing the oscillating effect imparted by the selection levers thereby causing possible errors in selecting the needles as well as breakage of the selectors. Another disadvantage related to the use of electromagnets to move the selection levers resides in the fact that in order to achieve satisfactory safety and activation speed the actuation of the electromagnets requires relatively high initial acceleration voltages, with consequent overheating and wear of the electromagnets.